通信网05-Chapter5-2

Problems using sliding window for both error & flow control
Choice of window size 窗口大小的选择问题 Interplay between transmission rate & retransmissions TCP separates error & flow control
Arrival times
Time
Playout clock must be synchronized to transmitter clock
Send times Playout times
播放时钟必须与发送 时钟同步
Tplayout Receiver too fast buffer starvation
TCP ARQ Method
• TCP uses Selective Repeat ARQ 选择重传ARQ
fn
Df=fn-fs=fn-(M/N)fn
N ticks
fr=fn-Df

Clock recovery simple if a common clock is available to transmitter & receiver


Transmitter sends Df of its frequency & network frequency Receiver adjusts network frequency by Df Packet delay jitter can be removed completely
捕获到脉搏后，持 续测试60秒
记录结果，画出曲线， 可记录到云端，或分享 到facebook
通过外设（蓝牙/接线），可以对 血压、血糖等体征进行测试 一个外设50~100美元
非常典型的生物科学与信息科学的融合创新！
当我们热衷于为Iphone设计各种各样的手机套的时候， （红木手机壳、竹子手机壳…..）美国人在干什么？
Receiver window starts at {0, …, Wr} Window slides forward to从1开始的 {Ws,…,Ws+Wr-1} Receiver rejects frame 0 because it is outside receive window

2m-1
0 Slast


Mismatch between sending rate & rate at which user can retrieve data Surges in frame arrivals

Flow control prevents buffer overflow by regulating rate at which source is allowed to send information



ACKs that slide window forward can be viewed as permits to transmit more 可以被看做允许传送 Can also pace ACKs as shown above

Return permits (ACKs) at end of cycle regulates transmission rate
1 2
2m-1 Ws +Wr-1
0
1 2
send window
receive window
Rnext Ws
Ws-1
技术创新之 health online
从一个新的Iphone应用 heartfitness讲起。
heartfitness
将手指贴到摄像头 上，遮住闪光灯
工作原理（只能测试心率）： 1、闪光灯照射手指，半透光效果； 2、摄像头捕获手指内血管的脉动； 3、CPU处理脉动信号，尝试同步； 4、CPU开始对脉搏计数，得到心率； 5、连接云平台或SNS平台； 6、简单、便捷、易用、精准；
X ON / X OFF 开关闸门模式
threshold
Information frame
Transmitter Receiver Transmit A on off on off
X OFF
Transmit
Time
B
2Tprop
Time
Threshold must activate OFF signal while 2 Tprop R bits still remain in buffer
Synchronization to a Common Clock 同步到网络时钟
Transmitter M fs M=#ticks in local clock In time that net clock does N ticks Network Receiver M fr
fn/fs=N/M
N ticks Network clock
Receiver too slow; buffer fills and overflows
time
Time
Receiver speed Time just right
Many late packets
Tplayout
time
Tplayout
time
Clock Recovery 时钟恢复
Timestamps inserted in packet payloads indicate when info was produced
Flow Control
Flow Control 流量控制
buffer fill
Information frame
Transmitter Receiver
Control frame


Receiver has limited buffering to store arriving frames接收者的存储有限 Several situations cause buffer overflow
Synchronous source sends periodic information blocks
Network output not periodic
Network



Applications that involve voice, audio, or video can generate a synchronous information stream Information carried by equally-spaced fixed-length packets Network multiplexing & switching introduces random delays
Why Ws + Wr <= 2m works

Transmitter sends frames 0 to Ws-1; send window empty All arrive at receiver All ACKs lost Transmitter resends frame 0

+ t4 t3 t2 t1 Counter

Buffer for information blocks
Error signal Smoothing Add filter
Playout command Adjust frequency Recovered clock
Timestamps
Counter attempts to replicate transmitter clock 计数器用于模拟发送者的时钟 Frequency of counter is adjusted according to arriving timestamps 时间戳来自发 送方的计数器，计算计数器差值的变化调整频率 Jitter introduced by network causes fluctuations in buffer & in local clock 时延抖 动会影响时钟
思考题：ARQ同时用于差错控制 和流量控制会有什么问题？
误码率
高误码 低速
高误码 高速
信道速率
低误码 低速 低误码 高速
Chapter 5 Peer-to-Peer Protocols and Data Link Layer
Timing Recovery
定时恢复
Timing Recovery for Synchronous Services 同步服务的定时恢复
课前提问
1. 请比较传输层和数据链路层提供的复用功能 2. 有了逐跳方式的数据链路层差错控制，是否还需要传输层 端到端的差错控制？为什么？举生活中的例子说明。 3. 为什么停等协议中的序号用1比特就够了 4. 选择重传ARQ中，NAK的作用是什么？只用ACK行不行？ 为什么？ 5. GBN ARQ和选择重传ARQ的窗口尺寸有什么要求？为何窗 口尺寸会有限制？ 6. GBN ARQ的发送窗口是不是一次只能向前滑动一格？
Window Flow Control 窗口模式
Return of permits tcycle A
Time
B
Time

Sliding Window ARQ method with Ws equal to buffer available

Transmitter can never send more than Ws frames
Hale Waihona Puke 血压计体重计健康眼镜
Baby体重计
睡眠测试/心电跟踪
受伤恢复指导
我们利用了Iphone的流行，美国人利用了Iphone提供的online功能对接云平台。